Thursday, December 28, 2017

Some Recent Physics Experiment Results

* The ATLAS experiment at the LHC continues to exclude the possibility of light supersymmetric particles.

The search for strongly produced supersymmetric particles decaying into final states with multiple energetic jets and either two leptons (electrons or muons) with the same electric charge or at least three leptons was performed using the proton-proton collision dataset of 36 fb−1 at 13 TeV recorded with the ATLAS detector in 2015 and 2016. Due to the low Standard Model background, these final states are particularly adapted to searches for gluinos or third generation squarks in several supersymmetric production topologies determined from a variety of simplified and phenomenological models. The absence of excess over the Standard Model prediction is interpreted in terms of limits on the masses of superpartners derived at 95% confidence level. In the studied decay modes and depending on the decay topology, the existence of gluinos with masses below 1.9 TeV, sbottoms with masses below 700 GeV, and neutralinos with masses below 1.2 TeV are excluded.

* Production of a Higgs boson with a top quark pair has been seen with a significance of 4.2 sigma. The results are consistent with the Standard Model prediction.

"An excess of events over the expected background from Standard Model processes is found with an observed significance of 4.1 standard deviations, compared to an expectation of 2.8 standard deviations. The best fit for the tt¯H production cross section is σ(tt¯H)=790+230−210 fb, in agreement with the Standard Model prediction of 507+35−50 fb. The combination of this result with other tt¯H searches from the ATLAS experiment using the Higgs boson decay modes to bb¯, γγ and ZZ∗→4ℓ, has an observed significance of 4.2 standard deviations, compared to an expectation of 3.8 standard deviations. This provides evidence for the tt¯Hproduction model."

* The Higgs boson observed at the LHC continues to be consistent with the Standard Model Higgs boson according to the ATLAS experiment.

"For a Higgs boson mass of 125 GeV, the ratio of the measured tt¯H signal cross-section to the Standard Model expectation is found to be μ=0.84+0.64−0.61. A value of μ greater than 2.0 is excluded at 95% confidence level while the expected upper limit is μ<1.2 in the absence of a tt¯H signal."

From here.* As predicted there is still no experimental evidence of magnetic monopoles.* A current as of 2017 summary of top quark physics can be found in this review article (link to pdf). Notably, it explores the definitional difficulties with measuring the top quark mass, which based on the global average by the most precise available means is about 171.9 GeV to 174.3 GeV at plus or minus two sigma in direct measurements, which are consistent with less precise cross-section based measurements. The global average width measurement at plus or minus two sigma is 1.11 GeV to 1.79 GeV with a theoretically predicted value for a 172.5 GeV mass top quark of 1.322 GeV.* There is considerable variety in the properties of the different kinds of filaments of matter that stretch across the universe.